This invention relates to techniques for the contactless measurement using an electron (E) beam and it relates, more particularly, to an electron source for electron beam testers.
Electron beam testers typically operate at very low duty cycles wherein the electron beam is on for a very small fraction of the time that the circuit under test is being inspected. In such a case, the signal-to-noise ratio of electron beam testers is a function of the number of electrons collected. Hence, this is related to the total number of electrons that strike the point being inspected. The number of electrons is dependent upon the electron beam current of the column of electrons. For a conventional LaB.sub.6 source, the electron beam would typically have a very low current of about 1 nanoampere. Field emission sources produce currents several times larger.
For high speed circuits to be examined, it is necessary for the electron pulses to be very short. This allows observation of fast waveforms. In this situation, the output of the electron beam tester is the convolution of the electron beam pulse with the waveform under observation. When a one hundred picosecond pulse occurs in an electron beam current of one nanoampere, each electron pulse only contains on the order of one or two electrons. Such electron pulses have extremely low signal-to-noise ratios and require very long integration times to yield useable signals.
It is an object of the present invention to provide some form of electron storage to produce a significant increase in the effective beam current suitable for the combination of low duty cycles in pulses of short duration.